U.S. Pat. No. 4,127,038 discloses a sprocket shift assembly wherein a chain is moved from sprocket to sprocket, the sprockets being of different diameter, while maintaining a positive drive connection during shifting. The chain is shifted from a sprocket around which the chain is entrained to a receiving sprocket by pivoting a sector of the receiving sprocket so that it is aligned with the teeth of the chain entrained sprocket. After the chain has been shifted to the receiving sprocket the sector is returned to its original position with the chain entrained thereon.
U.S. Pat No. 4,580,997 discloses a two-speed sprocket and chain shift assembly. One sprocket has a small pivoted sprocket segment. A shifting mechanism shifts the chain, entrained on one sprocket, to another by pivoting the sprocket segment to align it with the chain. The patent also discloses the use of a pawl housing and pivotable cam to direct travel of a pawl carried by the sprocket as necessary to shifting or non-shifting of the sprocket and chain assembly.
U.S. Pat. No. 4,894,046 discloses a control unit for a chain shifter using a reversible electric selector motor to position a cam assembly within a pawl guide housing. A traveling pawl is thereby directed as necessary for chain shifting. A spring loaded latch key positions the cam selector. The spring biases left and right latch arms apart so that they hold the cam selector in a centered position. In operation, the motor must push one of two latch arms away from the latch key to permit the cam selector to rotate. The spring forces the latch members against the key and thus the motor must exert significant force to overcome the spring's force and move the latch key. The force required by the motor puts significant demands on the battery driving the motor. As a result, the battery must be frequently replaced. Alternatively, a larger battery may be used, however, a large and heavy battery is not suitable for use with a bicycle.
Moreover, the force output from a motor is not instantaneous, but rather, builds over time. The motor in the above system must push against one of the latch arms for a period of time until the motor exerts a force great enough to move the latch arm. Not only does this place further demands on the battery, but the time delay between activation of the motor and shifting of the chain may be undesirable.
Consequently, there is a need for an improved control unit for a chain shifter which has increased power efficiency. Additionally, there is a need for an improved control unit for a chain shifter which is capable of rapid positioning of the cam selector.